This invention relates generally to medical devices, and more particularly, to apparatus and methods for improved deployment of stents or other implantable medical devices.
Stents may be inserted into an anatomical vessel or duct for various purposes. Stents may maintain or restore patency in a formerly blocked or constricted passageway, for example, following a balloon angioplasty procedure. Other stents may be used for different procedures, for example, stents placed in or about a graft have been used to hold the graft in an open configuration to treat an aneurysm. Additionally, stents coupled to one or both ends of a graft may extend proximally or distally away from the graft to engage a healthy portion of a vessel wall away from a diseased portion of an aneurysm to provide endovascular graft fixation.
Stents may be either self-expanding or balloon-expandable, or they may have characteristics of both, depending on particular needs that are more suited to one stent type or the other. Self-expanding stents may be delivered to a target site in a patient's vascular system in a compressed configuration and subsequently expanded by removing a delivery sheath, removing trigger wires and/or releasing diameter reducing ties. Self-expanding stents expand primarily based on their own expansive force without the need for further mechanical expansion. In one example, a stent made of a shape-memory alloy such as Nitinol may allow the stent to return to a predetermined expanded configuration upon removal of a sheath or other device that maintains the stent in its compressed, pre-deployment configuration. In another example, stents made of materials such as stainless steel expand on their own accord once released from constraints holding them in their compressed state.
When an expandable stent is deployed, it is important to position it at the precise desired location within the lumen of a patient's vascular system. In some cases, as soon as sheath is withdrawn from the proximal end of the stent, the proximal end may expand in a rapid and irregular way, with the risk that one or more of the stent struts may be deformed, bent or damaged. Moreover, as the sheath slides over and beyond the distal end of the stent, the stent may move longitudinally out of position within the vessel as a result of the force of the sheath being pulled back as well as expand radially in a manner that may be difficult to control. As such, placement of the stent may be less accurate and may lead to damage of the vessel. Commonly, release devices such as trigger wires may be provided as a deployment control mechanism, which releasably couple the proximal and/or distal ends of a stent or stent-graft to a delivery catheter until the trigger wires are selectively released by the physician. Typically, one or more trigger wires are looped through a portion of the stent, such as the proximal stent apices, for example, to pull the stent closely (radially inward) against the delivery catheter. Trigger wires may also help prevent unwanted longitudinal movement of the stent during placement within a vessel and withdrawal of the sheath. Release of a trigger wire causes full radial expansion of the stent, such that the stent engages an inner wall of a duct or vessel. However, trigger wire systems can be complicated systems. The present invention presents a method and apparatus for the reduced diameter delivery of stent-grafts with controlled release and reduced deployment forces.